Four people have been arrested at Parc Prison over allegations of assault and misconduct.
Detectives they're investigating following reports of a series of incidents - and concerns about staff conduct - at the Bridgend jail on Wedneday.
Three men, aged 25, 35 and 45, from the Llanelli, Merthyr and Pontcymmer areas are being questioned, along with a 23 year-old woman from Bridgend.
South Wales Police has stressed its enquiries are at an early stage.
Detective Chief Inspector Dean Taylor said: "We received concerns about the conduct of staff at HMP Parc on Wednesday September 18."
"Officers are at the early stages of the investigation and are working closely with G4S while inquiries are ongoing."
Parc Prison has been at the centre of controversy in recent months.
13 inmates have died so far this year with at least four of the deaths being linked to drugs.
In June, its director Heather Whitehead resigned after just ten months in charge and was replaced by Will Styles.
But earlier this week, the social justice minister, Jane Hutt, insisted there had been "considerable progress" in the day-to-day running of the jail, describing the situation as "much, much more stable".
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